1. Field of the Invention
The present invention relates to a liquid crystal display element of optical writing type, which is preferably utilized in a liquid crystal display apparatus of projection type.
2. Description of the Related Arts
In a liquid crystal display apparatus of projection type such as a projection type TV (television), a liquid crystal display element of optical writing type is utilized as a liquid crystal light valve. The inventors of the present application know one kind of such a liquid crystal display element, which is manufactured as following.
Namely, a first transparent electrode is formed on a first glass substrate. On this first transparent electrode, a photoconductive layer is formed of a hydrogenated amorphous silicon (a-Si:H). On this photoconductive layer, a light absorbing layer is formed of a thin film of carbon or metal such as silver. On this light absorbing layer, a dielectric mirror is formed of multiple layered films of titanium dioxide/silicon dioxide, zinc sulfide/magnesium fluoride, for example. On this dielectric mirror, a first orientation film is formed of a polyimide film which undergoes a molecular orientation treatment.
On the other hand, a second transparent electrode is formed on a second glass substrate. On this second transparent electrode, a second orientation film is formed. The first and second glass substrates, on which these films are thus formed, are attached to each other through a spacer, and a liquid crystal is disposed between the first and second orientation films, so as to form the liquid crystal layer therebetween.
On each of the outer surfaces of the first and second glass substrates, a reflection preventing film is formed so as to prevent the reflection of these glass surfaces. For the operation of thus constructed liquid crystal display element, an alternating voltage source is connected between the first and second transparent electrodes.
In the operation, when a laser beam as a writing light, is inputted to the liquid crystal display element from the side of the first glass substrate, the impedance of the photoconductive layer is reduced at an area which receives the laser beam. Thus, the orientation of the liquid crystal molecule is changed at this area since the voltage is applied to this area of the liquid crystal layer by the alternating voltage source.
At other area of the photoconductive layer which does not receive the laser beam, the impedance of the photoconductive layer is not changed, so that the orientation of the liquid crystal at this area is kept constant as its original state. As a result, an image is written in the liquid crystal display element according to the orientation change of each portion of the liquid crystal layer.
Then, a projection light i.e. a light for reading out the image written in the liquid crystal layer, is inputted to the liquid crystal display element from the side of the second glass substrate through a light polarization plate, so that the reflection light of the projection light, which is reflected at the dielectric mirror and which polarization direction is selectively changed by the liquid crystal layer, is projected onto the screen after passing through the light polarization plate again.
In the above mentioned case, the dielectric mirror is provided in the liquid crystal display element, with a purpose to reflect the projection light toward the screen and, at the same time, to prevent the projection light from entering the photoconductive layer by reflecting the projection light with a high reflection coefficient, so as not to change the impedance of the photoconductive layer by the projection light.
On the other hand, the light absorbing layer is provided in the liquid crystal display element, with a purpose to prevent the laser beam from reflecting again toward the photoconductive layer and, at the same time, to shut the projection light which has passed through the dielectric mirror.
As described above, the light reflecting layer i.e. the dielectric mirror, is indispensable in the liquid crystal display element of optical writing type when it is utilized in the liquid crystal display apparatus of projection type, and that, the dielectric mirror, which is made of insulation material to be reduced in conductivity, is utilized as this light reflecting layer. Here, the dielectric mirror is formed by vapor-depositing multiple layers of insulation films.
However, the vapor-depositing technique to form the multiple layers of insulation films, is rather complicated and difficult, with bringing a drawback that the overall manufacturing processes of the liquid crystal display element are made quite complicated and time and cost consuming, besides the cell structure itself of the liquid crystal display element, is made quite complicated.